Argumentum ad hominem (argument directed at the person). This is the error of attacking the character or motives of a person who has stated an idea, rather than the idea itself. The most obvious example of this fallacy is when one debater maligns the character of another debater (e.g, "The members of the opposition are a couple of fascists!"), but this is actually not that common. A more typical manifestation of argumentum ad hominem is attacking a source of information -- for example, responding to a quotation from Richard Nixon on the subject of free trade with China by saying, "We all know Nixon was a liar and a cheat, so why should we believe anything he says?" Argumentum ad hominem also occurs when someone's arguments are discounted merely because they stand to benefit from the policy they advocate -- such as Bill Gates arguing against antitrust, rich people arguing for lower taxes, white people arguing against affirmative action, minorities arguing for affirmative action, etc. In all of these cases, the relevant question is not who makes the argument, but whether the argument is valid.I think we've already seen these deployed.
It is always bad form to use the fallacy of argumentum ad hominem. But there are some cases when it is not really a fallacy, such as when one needs to evaluate the truth of factual statements (as opposed to lines of argument or statements of value) made by interested parties. If someone has an incentive to lie about something, then it would be naive to accept his statements about that subject without question. It is also possible to restate many ad hominem arguments so as to redirect them toward ideas rather than people, such as by replacing "My opponents are fascists" with "My opponents' arguments are fascist."
Argumentum ad ignorantiam (argument to ignorance). This is the fallacy of assuming something is true simply because it hasn't been proven false. For example, someone might argue that global warming is certainly occurring because nobody has demonstrated conclusively that it is not. But failing to prove the global warming theory false is not the same as proving it true.
Whether or not an argumentum ad ignorantiam is really fallacious depends crucially upon the burden of proof. In an American courtroom, where the burden of proof rests with the prosecution, it would be fallacious for the prosecution to argue, "The defendant has no alibi, therefore he must have committed the crime." But it would be perfectly valid for the defense to argue, "The prosecution has not proven the defendant committed the crime, therefore you should declare him not guilty." Both statements have the form of an argumentum ad ignorantiam; the difference is the burden of proof.
In debate, the proposing team in a debate round is usually (but not always) assumed to have the burden of proof, which means that if the team fails to prove the proposition to the satisfaction of the judge, the opposition wins. In a sense, the opposition team's case is assumed true until proven false. But the burden of proof can sometimes be shifted; for example, in some forms of debate, the proposing team can shift the burden of proof to the opposing team by presenting a prima facie case that would, in the absence of refutation, be sufficient to affirm the proposition. Still, the higher burden generally rests with the proposing team, which means that only the opposition is in a position to make an accusation of argumentum ad ignorantiam with respect to proving the proposition.
Argumentum ad misericordiam (argument or appeal to pity). The English translation pretty much says it all. Example: "Think of all the poor, starving Ethiopian children! How could we be so cruel as not to help them?" The problem with such an argument is that no amount of special pleading can make the impossible possible, the false true, the expensive costless, etc.
It is, of course, perfectly legitimate to point out the severity of a problem as part of the justification for adopting a proposed solution. The fallacy comes in when other aspects of the proposed solution (such as whether it is possible, how much it costs, who else might be harmed by adopting the policy) are ignored or responded to only with more impassioned pleas. You should not call your opposition down for committing this fallacy unless they rely on appeals to pity to the exclusion of the other necessary arguments. It is perfectly acceptable to use appeal to pity in order to argue that the benefits of the proposed policy are greater than they might at first appear (and hence capable of justifying larger costs).
Argumentum ad nauseam (argument to the point of disgust; i.e., by repitition). This is the fallacy of trying to prove something by saying it again and again. But no matter how many times you repeat something, it will not become any more or less true than it was in the first place. Of course, it is not a fallacy to state the truth again and again; what is fallacious is to expect the repitition alone to substitute for real arguments.
Nonetheless, this is a very popular fallacy in debate, and with good reason: the more times you say something, the more likely it is that the judge will remember it. The first thing they'll teach you in any public speaking course is that you should "Tell 'em what you're gonna tell 'em, then tell 'em, and then tell 'em what you told 'em." Unfortunately, some debaters think that's all there is to it, with no substantiation necessary! The appropriate time to mention argumentum ad nauseam in a debate round is when the other team has made some assertion, failed to justify it, and then stated it again and again. The Latin wording is particularly nice here, since it is evocative of what the opposition's assertions make you want to do: retch. "Sir, our opponents tell us drugs are wrong, drugs are wrong, drugs are wrong, again and again and again. But this argumentum ad nauseam can't and won't win this debate for them, because they've given us no justification for their bald assertions!"
Argumentum ad numerum (argument or appeal to numbers). This fallacy is the attempt to prove something by showing how many people think that it's true. But no matter how many people believe something, that doesn't necessarily make it true or right. Example: "At least 70% of all Americans support restrictions on access to abortions." Well, maybe 70% of Americans are wrong!
This fallacy is very similar to argumentum ad populum, the appeal to the people or to popularity. When a distinction is made between the two, ad populum is construed narrowly to designate an appeal to the opinions of people in the immediate vicinity, perhaps in hope of getting others (such as judges) to jump on the bandwagon, whereas ad numerum is used to designate appeals based purely on the number of people who hold a particular belief. The distinction is a fine one, and in general the terms can be used interchangeably in debate rounds. (I've found that ad populum has better rhetorical effect.)
Argumentum ad populum (argument or appeal to the public). This is the fallacy of trying to prove something by showing that the public agrees with you. For an example, see above. This fallacy is nearly identical to argumentum ad numerum, which you should see for more details.
UPDATE: For example, courtesy of James Taranto here:
New York Times columnist Paul Krugman weighs in with strong support for the Democrats’ campaign to discredit voters who dissent from ObamaCare. Even by the standards of former Enron advisers, however, Krugman displays an astonishing lack of self-awareness:Keeping score? Krugman engages in a little Argumentum ad hominem, doesn't he? With a little Argumentum ad ignorantiam tossed in for good measure.
There was a telling incident at a town hall held by Representative Gene Green, D-Tex. An activist turned to his fellow attendees and asked if they “oppose any form of socialized or government-run health care.” Nearly all did. Then Representative Green asked how many of those present were on Medicare. Almost half raised their hands.Now, people who don’t know that Medicare is a government program probably aren’t reacting to what President Obama is actually proposing. . .
The driving force behind the town hall mobs is probably the same cultural and racial anxiety that’s behind the “birther” movement, which denies Mr. Obama’s citizenship. Senator Dick Durbin has suggested that the birthers and the health care protesters are one and the same; we don’t know how many of the protesters are birthers, but it wouldn’t be surprising if it’s a substantial fraction.
And cynical political operators are exploiting that anxiety to further the economic interests of their backers.
Does this sound familiar? It should: it’s a strategy that has played a central role in American politics ever since Richard Nixon realized that he could advance Republican fortunes by appealing to the racial fears of working-class whites.
Many people hoped that last year’s election would mark the end of the “angry white voter” era in America. Indeed, voters who can be swayed by appeals to cultural and racial fear are a declining share of the electorate.
While offering no arguments in favor of ObamaCare, Krugman equates its opponents with birthers, and birthers with racists, thereby reducing dissent to “the racial fears of working-class whites.” If you buy this--a big if--then the merits of ObamaCare are irrelevant. It has to pass, lest the “angry white voter” prevail.
I love Krugman's inability or unwillingness to connect certain dots - he assumes, for example, that people who are already on Medicare who are opposed to socialized medicine can only be opposed to further socialized medicine because they are too stupid to know Medicare is a government program. But couldn't he also have assumed the opposite - that because they have experienced Medicare they don't want any more of it? And, if they have read the proposed plans concerning "cost benefit" analysis as it relates to potential treatment (or lack thereof) for their future illnesses then they are exactly the people who should be protesting the plan - meaning that they are a helluva lot brighter than Krugman.